ZURICH, Nov 16 (Reuters) - The U.S. Food and Drug Administration on Thursday approved Roche's Hemlibra, a new medicine for haemophilia A that the Swiss drugmaker is counting on to help to offset eroding sales of older medicines going off patent.

But the FDA slapped Roche's medicine with a black box warning, the most serious available, about the risk of blood clots, giving a boost to shares in rival Shire, whose products lead the $11 billion haemophilia drug market.

The U.S. regulator approved Roche's drug, previously known as ACE910, or emicizumab, as a once-weekly injection for adults and paediatric patients with haemophilia A who have developed inhibitors, or resistance, to other treatments.

Haemophilia is a bleeding disorder in which a clotting protein is missing or does not function normally. Roche plans to charge about $482,000 for the first year of treatment and $448,000 a year after that.

Hemlibra, which analysts polled by Reuters estimate will produce about $1.6 billion in annual sales by 2022, is a key pillar of Roche CEO Severin Schwan's plan to introduce new blockbusters to offset declining sales of older medicines.

"Today’s approval of Hemlibra represents an important advancement for people with haemophilia A with inhibitors, who have struggled to manage their bleeding disorder and haven’t had a new medicine in nearly 20 years,” said Sandra Horning, Roche’s chief medical officer.

The company added that it is happy the black box warning exists because it clarifies the circumstances under which serious adverse events occur and provides guidance on how to manage those events.

The FDA warning label stems from patients on Hemlibra who developed thrombosis, or blood clots, while being treated for so-called breakthrough bleeds that sometimes occur in haemophilia patents undergoing therapy.

Some analysts have said a less-than-pristine safety profile could hamper Roche's efforts to achieve speedy adoption of Hemlibra - and potentially preserve Shire's leadership in the field.

Success of Hemlibra is critical for Schwan to make good on his pledge to investors of continued sales growth as patent expirations start to bite.

Rituxan, Roche's best-selling medicine at 7.3 billion Swiss francs in 2016, registered a 16 percent drop in European sales in the third quarter as cheaper copies muscled in on its turf in arthritis and blood cancer.

Rituxan, Avastin and Herceptin together account for more than $20 billion in annual sales but all face patent expirations over the next three years.

During trials, Roche's new drug cut the bleed rate by 87 percent in patients who had developed resistance to standard treatment, compared with those who instead received so-called bypassing agents that are now standard therapy.

The Swiss drugmaker is still awaiting Phase III trial data on Hemlibra in patients who have not developed inhibitors. Success there could help it to expand those eligible for the drug.

Separately on Thursday, the FDA expanded approvals for Roche's Gazyva to include previously untreated follicular lymphoma, bolstering the Swiss drugmaker's efforts to strengthen its portfolio of blood cancer medicines.

Analysts expect Gazyva sales to reach 1.2 billion Swiss francs ($1.21 billion) a year by 2020, according to a Reuters poll, compared with 196 million francs last year. (Reporting by John Miller; Editing by David Goodman and Dan Grebler)